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Old 8th January 2013, 09:48 PM   #11
DF96 is offline DF96  England
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Details of grounding will affect the result. No amount of filtering will deal with a noisy ground.
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Old 8th January 2013, 10:41 PM   #12
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Check the effects of loading the SMPS at the input with resistors. Start low and go all the way up to current limit of PS.
It may take away those spikes completely.
Also dont try inductive filters without some resistive load.
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Old 9th January 2013, 01:20 AM   #13
Hengy is offline Hengy  Canada
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The size of the resistor R depends on how much current you are going to draw. The higher the value you can choose, the better the filtering.
knutn,
I thought about trying a RC filter, however, using a higher value resistor (> 8 Ohms) drops more than 5V at 700mA, so then the regulators don't have enough headroom. If I use smaller resistors, it would provide less than 5dB attenuation. I'm not sure if that would be enough
This was based on rough calculations, which came pretty close to an online calculator I found


rikkitikkitavi,
I will purchase some inductors and ferrite beads asap, and try those. Is there any reason not to try a larger inductor and/or cap?

I am curious though, if MHz ringing is what is causing the noise I hear on my headphones? I have heard of harmonics, and even studied a little bit about it in post-sec, but surely it is all well above the audible range? I would never be able to hear a MHz signal.


whizgeek,
Even if they do go away with a higher load, I have no justifiable means of continuously putting that load on the smps while using the headphone amp. (I am not one of those who like a blindingly bright, 50W blue LED on the front of their electronics! ) My regulators get warm enough without having more heat dissipated into the enclosure.
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Old 9th January 2013, 01:44 AM   #14
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I know this is not what you are asking (and I can't help with that as I have no experience with LC filters in Power Supplies) however there are a couple of things that I think need to be raised anyway

1. Your input to output differential is only 4.7V The LM317 performs best when the in to out differential is 5V or higher. How much difference that additional 0.3V will make though is probably academic. However if you reduce it further by doing a CRC then it *may* actually make the situation worse (even if the noise before the reg is lower!! certainly that was what I observed with my testing!

2. Using low esr caps on the output of an LM317 is a no-no unless you add some series resistance in yourself. I'd suggest bumping up the output cap to say 47uF with maybe around 0.5ohms in series with it. low esr cap on the output will most likely result in a resonance within the audio range.

Tony.
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Old 9th January 2013, 02:46 AM   #15
Hengy is offline Hengy  Canada
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Originally Posted by wintermute View Post
1. The LM317 performs best when the in to out differential is 5V or higher.
You learn something everyday! (and today was the first day of classes for the new semester, so I was getting worried )

I am planning on trying a LC filter first. It will reduce the voltage by less than 0.5V according to LTSpice.

Quote:
Originally Posted by wintermute View Post
2. Using low esr caps on the output of an LM317 is a no-no unless you add some series resistance in yourself.
I assumed low esr caps were better in every part of the power supply.


Thank you for your suggestions and ideas! I will definitely test them out at some point. I am interested to see if I can recreate both effects.

Hengy
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Old 9th January 2013, 03:37 AM   #16
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Originally Posted by Hengy View Post
You learn something everyday! (and today was the first day of classes for the new semester, so I was getting worried )

I am planning on trying a LC filter first. It will reduce the voltage by less than 0.5V according to LTSpice.



I assumed low esr caps were better in every part of the power supply.


Thank you for your suggestions and ideas! I will definitely test them out at some point. I am interested to see if I can recreate both effects.

Hengy
Careful with the low-ESRs -- the equivalent output impedance of yer typical 3 terminal regulator is inductive with a tiny resistance in series, so adding a low-ESR capacitor is a fine way to make a series resonant circuit. This is a case where the cheap part is a better choice.
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Old 9th January 2013, 09:11 AM   #17
SY is offline SY  United States
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Quote:
Originally Posted by DF96 View Post
Details of grounding will affect the result. No amount of filtering will deal with a noisy ground.
Do not ignore this in your quest for filtering! And not just grounding, general layout- HF signals have a tendency to couple in unexpected ways.

Additionally, you want to think about both differential and common mode noise.
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Old 9th January 2013, 09:52 AM   #18
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+1 to DF96 and SY. The point on the board you put your earth probe from your scope will make big differences to the apparent noise levels as well!! Try moving it around a bit and you should see

Additionally, I know this may sound silly, but try doing some measurements with the power turned off as well, just for a baseline. I discovered that there were all sorts of sources of the noise when testing my LM317 PS. Getting reliable measurements at the low levels you are trying is actually very difficult!

Turn off every device in the house that you can. I had noise at 833Khz (see attached pic) pre reg (at 60mV p2p) which turned out to be coming from a PC (or it's monitor) powersupply about 10M away!! Not everything may be coming from whence you think

Tony.
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Old 9th January 2013, 10:31 AM   #19
DF96 is offline DF96  England
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You may need to turn off local AM broadcast transmitters too.
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Old 9th January 2013, 10:34 AM   #20
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Yep!!! that was the other problem I had Try leaving the earth of your scope probe hanging in the air and touch the tip to your finger and see what you get

Tony.
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Last edited by wintermute; 9th January 2013 at 10:41 AM. Reason: looked up the post to see what I had done, and corrected ;)
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